If you had been more alert, this golden opportunity would not have __________

This sentence conveys regret over a missed opportunity due to a lack of alertness. Therefore, the blank should be filled with a phrase that expresses the idea of losing something important due to negligence or inattention.

The idiom "slipped through your fingers" (Option D) is the most appropriate in this situation since it conveys the idea of letting a significant opportunity pass unnoticed or ungrasped.

The other options do not work as well. Since "escape" is not idiomatically used with "fingers" in this sense, "Escaped your fingers" (Option A) is an incorrect and awkward construction. "Slipped off" (Option B) does not fit the metaphor of a missed opportunity because it typically refers to a real object falling off, such as a ring. Since "escape" is already an intransitive verb and does not need "away" to complete its meaning, "Escaped away" (Option C) is redundant and grammatically incorrect.

Option D is therefore the correct answer in this case.